1. Field of the Invention
This invention relates to an electrophotographic photosensitive member which can reduce image defects, has a high charging performance and can form good high-density images over a long period of time, and to a process for producing such an electrophotographic photosensitive member.
2. Related Background Art
Materials that form photoconductive layers in solid-state image pick-up devices or in electrophotographic light-receiving members in the field of image formation or in character readers are required to have properties as follows: They are highly sensitive, have a high SN ratio [photocurrent (Ip)/dark current (Id)], have absorption spectra suited to spectral characteristics of electromagnetic waves to be radiated, have a high response to light, have the desired dark resistance and are harmless to human bodies when used; and also, in the solid-state image pick-up devices, the materials are required to have properties that enable afterimages to be erased in a prescribed time. In particular, in the case of electrophotographic photosensitive members of electrophotographic apparatus used as business machines in offices, it is important that they are safe to use.
Materials that generate interest from such a viewpoint include amorphous silicon (hereinafter “a-Si”) whose dangling bonds have been modified with monovalent elements such as hydrogen or halogen atoms, and its application to electrophotographic photosensitive members is disclosed in, e.g., U.S. Pat. No. 4,265,991.
Many processes by which electrophotographic photosensitive members comprised of a-Si are formed on conductive supports, are known in the art, as exemplified by sputtering, a process in which source gases are decomposed by heat (thermal CVD), a process in which source gases are decomposed by light (photo-assisted CVD) and a process in which source gases are decomposed by plasma (plasma-assisted CVD). In particular, one having been put into practical use in a very advanced state at present is plasma-assisted CVD (chemical vapor deposition), i.e., a process in which source gases are decomposed by direct-current or high-frequency or microwave glow discharge to form deposited films on the conductive support.
For example, as the layer construction of such deposited films, there are proposed those in which a “surface layer” or an “upper-part blocking layer” having blocking power is further provided on the surface side, in addition to electrophotographic photosensitive members composed chiefly of a-Si and modification elements added appropriately, as conventionally practiced. For example, U.S. Pat. No. 6,090,513 discloses an electrophotographic photosensitive member provided between a photoconductive layer and a surface layer an intermediate layer (upper-part blocking layer) having carbon atoms in a smaller content than the surface layer and incorporated with atoms capable of controlling conductivity.
Such conventional processes for producing electrophotographic photosensitive members have made it possible to obtain electrophotographic photosensitive members having characteristics and uniformity which are practical to a certain extent. Strict cleaning of the interiors of vacuum reactors also makes it possible to obtain electrophotographic photosensitive members reducing defects to a certain extent. However, with such conventional processes for producing electrophotographic photosensitive members, there is an unsolved problem in that, for products in which large-area and relatively thick deposited films are required as in electrophotographic photosensitive members, it is difficult, e.g., to obtain in a high yield deposited films that have uniform film quality, can satisfy requirements for various optical and electrical properties and also can reduce image defects when images are formed by an electrophotographic process.
In particular, a-Si films have a disposition that, where any nuclei-forming matters such as dust in the order of micrometers have adhered to the support surface or deposited-film surface, the dust serves as nuclei during deposition to cause the growth of “protuberances”. FIG. 2 is a diagrammatic sectional view showing an example of such protuberances of a conventional electrophotographic photosensitive member. The photosensitive member shown therein is constituted of a support 201 having a conductive surface, and a photoconductive layer 202 and a surface layer 203 superposingly formed thereon. Inclusion of dust 204 in the course of forming this photoconductive layer 202 causes abnormal growth on the dust that serves as nuclei during the deposition of a film. Such protuberances 205 have the shape of reversed cones whose vertexes start from the nuclei, and have a disposition that they have a lower ability to retain electric charges than the normal area.
Hence, some part of the protuberances appears in the form of white dots in solid black images on images formed (in the case of reverse development, appears in the form of black dots in solid white images). This image defect called “dots” is put to severer standards year by year. Where electrophotographic photosensitive members are set in color copying machines, the standards come much severer. In order to lessen such nuclei of protuberances, supports to be used are strictly cleaned before deposition, where the steps of setting the supports in a reactor are all operated in a clean room or in vacuo. In this way, efforts have been made so as to lessen as far as possible the dust which may adhere to the support surface before the deposition is started, thus the desired effects have been obtained. However, the cause of the occurrence of protuberances is not limited to the dust having adhered to the support surface. That is, where a-Si electrophotographic photosensitive members are produced, the layer thickness required is as large as several micrometers to tens of micrometers, and hence the deposition time reaches several hours to tens of hours. During such deposition, the deposited film of the a-Si and powdery polysilane is deposited not only on the supports but also on inner walls of the reactor and structures inside the reactor.
These reactor inner walls and structures do not have any surfaces that have been controlled like the supports. Hence, depositions may weakly adhere to come off in some cases during deposition carried out over a long time. Once even slight depositions come off during deposition, they cause dust, and the dust adheres to the surfaces of photosensitive members under deposition, so that the abnormal growth takes place starting from the dust to cause protuberances. Accordingly, in order to maintain a high yield, careful control is required not only for supports before deposition but also for preventing depositions from coming off in the reactor during the deposition. This has made it difficult to produce the a-Si photosensitive members.